1. Field of the Invention
This invention relates to a combustible gas detecting element which changes its electrical resistance when in contact with a combustible gas, and also relates to a fabrication process for the preparation of the raw material of the detecting element.
2. Description of the Prior Art
As a rule, the methods for detecting a combustible gas by solid materials include two methods, one of which involves detecting a combustible gas by the temperature rise owing to its combustion on a catalyst by means of a resistor such as platinum wire, of which electric resistance is dependent upon the temperature. The other method involves detecting a combustible gas by determining the change in the electrical resistance of a semiconductor accompanied by the adsorption of gas thereon. The former gives an output in proportion to the concentration of gas and accordingly may be used principally for a gas concentration meter and the like. The latter can provide an inexpensive detecting means and accordingly may be used as a gas leak detector and the like.
The present invention provides a gas detecting element of semiconductor type for a combustable gas detecting means according to the latter method.
A gas responsive element is held in an atmosphere of high temperature because a gas detecting element of the semiconductor type requires, in general, a high-speed response. Thus the oxides which are stable in an oxidizing atmosphere are selected as gas responsive elements.
Recently considerable research and development on the materials for a combustible gas detecting element has been conducted, centered on metal oxide semiconductors. This is caused by the problem that explosions due to combustible gas and poisoning due to a noxious gas, like carbon monoxide, are frequently generated in the home and in a variety of factories.
Liquefied natural gas (LNG) containing methane gas as its principal component has come into wide use in homes in many countries. Thus, a gas detecting element for selectively detecting methane gas, which is the principal component of LNG, is also in remarkably great demand.
Of course, a gas detecting element for responding to methane gas has already been developed. However, such gas detecting elements have a variety of defects such as catalyst poisoning due to a variety of gases, low selectivity for methane gas, great dependence upon the ambient humidity and the like because they contain noble metal catalysts as the activators for the responsive material. Thus, they have no practical use as yet.
As to the noxious gas, especially like carbon monoxide, a detecting element having enough sensitivity and high stability for practical use has been required to be developed. However, a highly sensitive and stable detecting element to carbon monoxide has not been put into practice yet. The gas concentration to be detected should be as low as 100 ppm, which is approximately one tenth of that of the other combustible gases like methane, propane or hydrogen.
It is required that detecting elements having great sensitivity to these gases should be remarkably active because of their high chemical stability. Thus, the addition of noble metal catalysts to responsive materials, and the operation of responsive materials at considerably high temperature and the like have been employed to realize a great sensitivity.
Next, the gas sensitive properties of ferric oxide will be described. It was recently found that gamma-type ferric oxide (.gamma.-Fe.sub.2 O.sub.3) having a spinel-type crystalline structure has excellent gas detection characteristics. There are various crystalline structures of ferric oxides which are very different from each other in their chemical and physical properties. Among the best known one is alpha-type ferric oxide (.alpha.-Fe.sub.2 O.sub.3) having a corundum-type crystalline structure. Besides, .gamma.-Fe.sub.2 O.sub.3, .beta.-Fe.sub.2 O.sub.3, .delta.-Fe.sub.2 O.sub.3, etc. are known. Among them, only the .gamma.-Fe.sub.2 O.sub.3 has actually useful gas detection characteristics.
Although .gamma.-Fe.sub.2 O.sub.3 has a large sensitivity to hydrogen, ethane, propane and iso-butane, its sensitivity to methane is not always sufficient.
Also .alpha.-Fe.sub.2 O.sub.3 has a remarkably small sensitivity not only to methane but also to ethane, propane, iso-butane and carbon monoxide, if the sensitive materials are manufactured from the commercial materials. That is to say the conventional ferric oxides by themselves do not have a sufficiently large sensitivity to combustible gases.
However we found a large sensitivity to combustible gases such as methane in .alpha.-Fe.sub.2 O.sub.3 composed of amorphous phase. In spite of its large sensitivity, the amorphous phase .alpha.-Fe.sub.2 O.sub.3 does not have sufficient stability and durability in life characteristics because of the degradation caused by crystallization of the amorphous phase, and also is insensitive to carbon monoxide.
Furthermore, by conventional wet processes of hydrolysis and precipitation for preparation of .alpha.-Fe.sub.2 O.sub.3, nonuniformity of the precipitated crystalline particle size and congelation of the particles are unavoidable, because of microscopic non-uniformity of the precipitant concentration in the solution. Consequently these cause instability in life characteristics and short durability.